.\" SPDX-License-Identifier: Linux-man-pages-1-para
.\"
.\" This man page is Copyright (C) 1999 Andi Kleen <ak@muc.de>.
.\"
.\" $Id: udp.7,v 1.7 2000/01/22 01:55:05 freitag Exp $
.\"
.TH udp 7 2024-05-02 "Linux man-pages 6.9.1"
.SH NAME
udp \- User Datagram Protocol for IPv4
.SH SYNOPSIS
.nf
.B #include <sys/socket.h>
.B #include <netinet/in.h>
.B #include <netinet/udp.h>
.P
.IB udp_socket " = socket(AF_INET, SOCK_DGRAM, 0);"
.fi
.SH DESCRIPTION
This is an implementation of the User Datagram Protocol
described in RFC\ 768.
It implements a connectionless, unreliable datagram packet service.
Packets may be reordered or duplicated before they arrive.
UDP generates and checks checksums to catch transmission errors.
.P
When a UDP socket is created,
its local and remote addresses are unspecified.
Datagrams can be sent immediately using
.BR sendto (2)
or
.BR sendmsg (2)
with a valid destination address as an argument.
When
.BR connect (2)
is called on the socket, the default destination address is set and
datagrams can now be sent using
.BR send (2)
or
.BR write (2)
without specifying a destination address.
It is still possible to send to other destinations by passing an
address to
.BR sendto (2)
or
.BR sendmsg (2).
In order to receive packets, the socket can be bound to a local
address first by using
.BR bind (2).
Otherwise, the socket layer will automatically assign
a free local port out of the range defined by
.I /proc/sys/net/ipv4/ip_local_port_range
and bind the socket to
.BR INADDR_ANY .
.P
All receive operations return only one packet.
When the packet is smaller than the passed buffer, only that much
data is returned; when it is bigger, the packet is truncated and the
.B MSG_TRUNC
flag is set.
.B MSG_WAITALL
is not supported.
.P
IP options may be sent or received using the socket options described in
.BR ip (7).
They are processed by the kernel only when the appropriate
.I /proc
parameter
is enabled (but still passed to the user even when it is turned off).
See
.BR ip (7).
.P
When the
.B MSG_DONTROUTE
flag is set on sending, the destination address must refer to a local
interface address and the packet is sent only to that interface.
.P
By default, Linux UDP does path MTU (Maximum Transmission Unit) discovery.
This means the kernel
will keep track of the MTU to a specific target IP address and return
.B EMSGSIZE
when a UDP packet write exceeds it.
When this happens, the application should decrease the packet size.
Path MTU discovery can be also turned off using the
.B IP_MTU_DISCOVER
socket option or the
.I /proc/sys/net/ipv4/ip_no_pmtu_disc
file; see
.BR ip (7)
for details.
When turned off, UDP will fragment outgoing UDP packets
that exceed the interface MTU.
However, disabling it is not recommended
for performance and reliability reasons.
.SS Address format
UDP uses the IPv4
.I sockaddr_in
address format described in
.BR ip (7).
.SS Error handling
All fatal errors will be passed to the user as an error return even
when the socket is not connected.
This includes asynchronous errors
received from the network.
You may get an error for an earlier packet
that was sent on the same socket.
This behavior differs from many other BSD socket implementations
which don't pass any errors unless the socket is connected.
Linux's behavior is mandated by
.BR RFC\ 1122 .
.P
For compatibility with legacy code, in Linux 2.0 and 2.2
it was possible to set the
.B SO_BSDCOMPAT
.B SOL_SOCKET
option to receive remote errors only when the socket has been
connected (except for
.B EPROTO
and
.BR EMSGSIZE ).
Locally generated errors are always passed.
Support for this socket option was removed in later kernels; see
.BR socket (7)
for further information.
.P
When the
.B IP_RECVERR
option is enabled, all errors are stored in the socket error queue,
and can be received by
.BR recvmsg (2)
with the
.B MSG_ERRQUEUE
flag set.
.SS /proc interfaces
System-wide UDP parameter settings can be accessed by files in the directory
.IR /proc/sys/net/ipv4/ .
.TP
.IR udp_mem " (since Linux 2.6.25)"
This is a vector of three integers governing the number
of pages allowed for queueing by all UDP sockets.
.RS
.TP
.I min
Below this number of pages, UDP is not bothered about its
memory appetite.
When the amount of memory allocated by UDP exceeds
this number, UDP starts to moderate memory usage.
.TP
.I pressure
This value was introduced to follow the format of
.I tcp_mem
(see
.BR tcp (7)).
.TP
.I max
Number of pages allowed for queueing by all UDP sockets.
.RE
.IP
Defaults values for these three items are
calculated at boot time from the amount of available memory.
.TP
.IR udp_rmem_min " (integer; default value: PAGE_SIZE; since Linux 2.6.25)"
Minimal size, in bytes, of receive buffers used by UDP sockets in moderation.
Each UDP socket is able to use the size for receiving data,
even if total pages of UDP sockets exceed
.I udp_mem
pressure.
.TP
.IR udp_wmem_min " (integer; default value: PAGE_SIZE; since Linux 2.6.25)"
Minimal size, in bytes, of send buffer used by UDP sockets in moderation.
Each UDP socket is able to use the size for sending data,
even if total pages of UDP sockets exceed
.I udp_mem
pressure.
.SS Socket options
To set or get a UDP socket option, call
.BR getsockopt (2)
to read or
.BR setsockopt (2)
to write the option with the option level argument set to
.BR IPPROTO_UDP .
Unless otherwise noted,
.I optval
is a pointer to an
.IR int .
.P
Following is a list of UDP-specific socket options.
For details of some other socket options that are also applicable
for UDP sockets, see
.BR socket (7).
.TP
.BR UDP_CORK " (since Linux 2.5.44)"
If this option is enabled, then all data output on this socket
is accumulated into a single datagram that is transmitted when
the option is disabled.
This option should not be used in code intended to be
portable.
.\" FIXME document UDP_ENCAP (new in Linux 2.5.67)
.\" From include/linux/udp.h:
.\"     UDP_ENCAP_ESPINUDP_NON_IKE draft-ietf-ipsec-nat-t-ike-00/01
.\"     UDP_ENCAP_ESPINUDP draft-ietf-ipsec-udp-encaps-06
.\"     UDP_ENCAP_L2TPINUDP rfc2661
.\" FIXME Document UDP_NO_CHECK6_TX and UDP_NO_CHECK6_RX, added in Linux 3.16
.TP
.BR UDP_SEGMENT " (since Linux 4.18)"
Enables UDP segmentation offload.
Segmentation offload reduces
.BR send (2)
cost by transferring multiple datagrams worth of data
as a single large packet through the kernel transmit path,
even when that exceeds MTU.
As late as possible,
the large packet is split by segment size into a series of datagrams.
This segmentation offload step is deferred to hardware if supported,
else performed in software.
This option takes a value in the range
.RB [ 0 ,\~ USHRT_MAX ]
that sets the segment size:
the size of datagram payload,
excluding the UDP header.
The segment size must be chosen such that
at most 64 datagrams are sent in a single call
and that the datagrams after segmentation meet
the same MTU rules that apply to datagrams sent without this option.
Segmentation offload depends on checksum offload,
as datagram checksums are computed after segmentation.
The option may also be set for individual
.BR sendmsg (2)
calls by passing it as a
.BR cmsg (3).
A value of zero disables the feature.
This option should not be used in code intended to be portable.
.TP
.BR UDP_GRO " (since Linux 5.0)"
Enables UDP receive offload.
If enabled,
the socket may receive multiple datagrams worth of data
as a single large buffer,
together with a
.BR cmsg (3)
that holds the segment size.
This option is the inverse of segmentation offload.
It reduces receive cost by handling multiple datagrams worth of data
as a single large packet in the kernel receive path,
even when that exceeds MTU.
This option should not be used in code intended to be portable.
.SS Ioctls
These ioctls can be accessed using
.BR ioctl (2).
The correct syntax is:
.P
.RS
.nf
.BI int " value";
.IB error " = ioctl(" udp_socket ", " ioctl_type ", &" value ");"
.fi
.RE
.TP
.BR FIONREAD " (" SIOCINQ )
Gets a pointer to an integer as argument.
Returns the size of the next pending datagram in the integer in bytes,
or 0 when no datagram is pending.
.B Warning:
Using
.BR FIONREAD ,
it is impossible to distinguish the case where no datagram is pending
from the case where the next pending datagram contains zero bytes of data.
It is safer to use
.BR select (2),
.BR poll (2),
or
.BR epoll (7)
to distinguish these cases.
.\" See http://www.securiteam.com/unixfocus/5KP0I15IKO.html
.\" "GNUnet DoS (UDP Socket Unreachable)", 14 May 2006
.TP
.BR TIOCOUTQ " (" SIOCOUTQ )
Returns the number of data bytes in the local send queue.
Supported only with Linux 2.4 and above.
.P
In addition, all ioctls documented in
.BR ip (7)
and
.BR socket (7)
are supported.
.SH ERRORS
All errors documented for
.BR socket (7)
or
.BR ip (7)
may be returned by a send or receive on a UDP socket.
.TP
.B ECONNREFUSED
No receiver was associated with the destination address.
This might be caused by a previous packet sent over the socket.
.SH VERSIONS
.B IP_RECVERR
is a new feature in Linux 2.2.
.\" .SH CREDITS
.\" This man page was written by Andi Kleen.
.SH SEE ALSO
.BR ip (7),
.BR raw (7),
.BR socket (7),
.BR udplite (7)
.P
The kernel source file
.IR Documentation/networking/ip\-sysctl.txt .
.P
RFC\ 768 for the User Datagram Protocol.
.br
RFC\ 1122 for the host requirements.
.br
RFC\ 1191 for a description of path MTU discovery.
